The invention relates to a power current cable system for the transmission of energy at very high power, with the energy being furnished by a current generator connected at one end of the cable system and being delivered to a load connected at the other end of the cable system, with the load carrying an inductive current component.
Transmission lines, cables, overhead power lines, and the like, all have finite maximum transmission lengths. The maximum transmission length for a particular type of transmission cable depends firstly upon the type of transmission cable involved, upon the stability criteria of the current generator, the difference between the load voltage and the generator voltage, and also the difference between the load current and the generator current. With overhead or open-air transmission lines, the maximum transmission length can be determined from each of the foregoing criteria; however, with cable-type transmission systems the maximum transmission length is predominantly dependent upon one of the last-mentioned two factors.
Very-high-power cables, such as cryogenic cables, superconductive cables, and pressurized-gas (SF.sub.6) cables as described for instance in Cryogenics (1969) pp. 165-176, IEEE Trans. PAS-89 (1970) pp. 1995-2003 and IEEE Trans. PAS-88 (1969) pp 369-375 respectively have relatively short maximum transmission lengths, and this greatly limits the usefulness and applicability of such cables. The limitation upon the transmission lengths of such cables derives from the fact that such cables are furnished, at their generator ends, with power in excess of their natural power. Natural power also known as natural load or natural loading is the power which a transmission cable is capable of furnishing to a matched load, i.e., a load having an impedance equal to the characteristic impedance of the transmission cable. For example, assume that a 110 kV cable has a characteristic impedance Z.sub.w. If this cable is furnishing power to a matched load, i.e., a load having an impedance equal to Z.sub.w, then the power which the cable is capable of furnishing to the load is evidently equal to P.sub.N = (110 kV).sup.2 /Z.sub.w. When the power transmitted over the cable has such values, there is a very significant distributed inductive voltage drop along the length of the cable. The greater the length of the cable, the greater is the voltage discrepancy between the voltage at the load and generator ends of the cable. Since the load usually exhibits inductive reactance, the voltage at the generator side of the cable will always be greater than the voltage at the load side of the cable. The voltage furnished by the generator is customarily made greater than that actually required by the load, to take losses into account. However, there is a limit to the voltage which can be safely applied to the generator end of the cable, and accordingly the safe transmission length of the cable becomes limited by the need to avoid the transmission of higher power than the cable can handle.
It is already known to attempt to increase the maximum transmission lengths of heavy duty cable, such as oil-filled cables, by connecting to the cables various reactive electrical components, namely condensors and inductors. This prior-art technique has had some success when the transmission of medium to high power was involved. However, with the transmission of very high power, for example 800-900 MVA, it becomes practically impossible to assign to the compensating energy-storing components circuit values that will satisfactorily take into account the high loading (voltage spikes) to be dealt with, so that the use of this kind of cable is still limited as to the maximum transmission length practicable.